Control of trihalomethanes in water treatment

ABSTRACT

Trihalomethanes (THM&#39;s) in water, particularly in drinking water, are substantially reduced by treatment of the water with small amounts, i.e., below about 50 ppm, of sulfite ion supplied by sodium metabisulfite or sulfur dioxide. The addition to the water of the sodium metabisulfite alters the structure of the THM&#39;s precursor and lowering the chlorine demand. The sulfite ion may be introduced prior to chlorination, or at an intermediate stage of the chlorination, or in adjunct with the chlorination which permits the use of substantially reduced amounts of chlorination.

TECHNICAL FIELD

The present invention relates to the chemical treatment of water,particularly potable water to effect a reduction in the formation oftotal trihalomethanes (TTHM's) in the water by introducing into thewater a small but effective amount of sulfite ion from a compoundselected from the group consisting of sodium metabisulfite and sulfurdioxide. Specifically, the treated water is processed so as to be incompliance with, and preferably well below, the Environmental ProtectionAgency's proposed maximum tolerable contamination level of TTHM's of onehundred parts per billion.

BACKGROUND OF THE INVENTION

Considerable publicity has been generated concerning the use, and harmtherefrom, of chlorine in drinking water plants resulting in theformation of the chlorinated hydrocarbons known as total trihalomethane.These are now labeled as carcinogenic by the EPA, which is expected toissue regulations which limit the TTHM at 100 ppb. Among the carcinogensformed are chloroform, carbon tetrachloride and trihalomethanes.

Many municipal drinking water plants are, or have been, involved inextensive bench tests and plant trials for methods to reduce the TTHM intheir plant water. The EPA and the American Water Works Association(AWWA) have expended considerable resources on consulting fees andfunding projects to this end. The alternative methods have been poor atbest, and purport to introduce very costly chemicals, or equipment forTHM reduction.

Some prior efforts to reduce total trihalomethane as summarized asfollows:

    ______________________________________                                        Means Attempted    Remarks                                                    ______________________________________                                        1.  Substituting chlorine with                                                                       (a)   Very expensive equip-                                chlorine dioxide         ment; over $1.0MM                                    Tests have shown that in for a medium size                                    some waters ClO.sub.2 is not                                                                           plant                                                always effective. It has                                                                         (b)   Chlorine & sodium                                    demonstrated an ability  chlorite are expen-                                  to reduce the halogens, but                                                                            sive chemicals.                                      at times, has no effect on                                                                       (c)   ClO.sub.2 is 15 to 20 ×                        the THM.                 cost of Cl.sub.2.                                                       (d)   Forms a toxic products                                                        of chlorite & chlor-                                                          ate in H.sub.2 O.                                                       (e)   ClO.sub.2 is a very un-                                                       stable explosive gas                                                          for a water treatment                                                         plant to store and                                                            handle. High energy                                                           (electrical) costs                                                            to operate in ore                                                             site chlorine dio-                                                            xide generator.                                  2.  Use of Powdered Activitated                                                                      (a)   Very expensive                                       Carbon (PAC)             equipment; over                                      PAC carbon columns require                                                                             $1.5MM for a medium                                  a furnace for regeneration.                                                                            size plant.                                          It is an added capital                                                                           (b)   Little effect after                                  expenditure, and the energy                                                                            chlorinated carbons                                  costs (oil, or natural gas)                                                                            form.                                                are astronomical.  (c)   Will not meet EPA's                                                           Maximum Trihalometh-                                                          ane Potential Limit,                                                          i.e., MPT = after 30                                                          hrs. retention, THM                                                           will increase.                                                          (e)   Unrealistic high                                                              dosages makes it an                                                           unlikely candidate.                              3.  Adding Ammonia with                                                                              (a)   Generally undesirable                                Chlorine                 and needs special                                    To form a more           feeding equipment.                                   stable disinfectant in                                                                           (b)   A tight chlorine to                                  the production of chlor- ammonia ratio (1 to                                  amines. Turbidity & pH   3) must be maintained                                of water is very sensitive                                                                             in system.                                           to success.        (c)   Treatment error will                                                          cause taste and odor                                                          problems.                                                               (d)   Not especially effec-                                                         tive against the THM,                                                         chloroform.                                                             (e)   A pH below 6.0 must                                                           be maintained until                                                           after post chlorina-                                                          tion. Causes "etching"                                                        on the cement walls                                                           of the sedimentation                                                          basins on the cement                                                          walls.                                           4.  Overdosing Treatment with                                                                        (a)   Reduced TOC by 60%                                   Alum                     but precurses not                                    Dosage rates, over those removed.                                             required for color & tur-                                                                        (b)   Significant treat-                                   bidity removal, were em- ment cost increases                                  ployed in the absence of dramatically.                                        prechlorination. Post                                                                            (c)   High demand on caus-                                 chlorination dosages were                                                                              tic soda for pH ele-                                 high.                    vation.                                                                 (d)   Creation of abnor-                                                            mally high sludge                                                             production.                                                             (e)   Low pH would etch                                                             cement sedimentation                                                          basins.                                          ______________________________________                                    

Other tests, includings, boiling, ozone, ultra violet light, iodine,surfactants, polymers, gamma radiation and others have also beenconsidered but because of cost, ineffectiveness or toxicity, such testswere abandoned.

It is thus apparent that a need exists for a practical and dependablemeans to effect the reduction of trihalomethanes in drinking water.

BRIEF SUMMARY OF THE INVENTION

This invention relates to a chemical process of controlling or reducingthe formation of the total trihalomethanes in the drinking watertreatment by applying a small but effective amount of a sulfite ionsupplying compound selected from sodium metabisulfite or sulfur dioxideto reduce the maximum contaminant level (MCL) of total trihalomethanes(TTHM) that would be present in the water.

More particularly, the invention relates to a chemical process ofreducing the chlorine demand required for drinking water as adisinfectant with the addition, during the purification treatment of thewater, of a small quantity of sodium metabisulfite or sulfur dioxide,preferably sodium metabisulfite. The sulfite ion is introducedpreferably prior to prechlorination, or intermediate chlorination stagesand may be employed with diminished amounts of prechlorination.

In a broader aspect the invention relates to the control of theprecursor or to the inhibition of an environment, in the water beingtreated so that it is nonconducive to the formation of trihalomethanesTHM by the use of sulfite ion; this frequently results in lower chlorinedemand because of the alteration in the structure of these precursorspresent in the water. Thus, the treatment of source waters to reducelevels of total trihalomethane (TTHM) precursors prior to chlorinationis an attractive alternative to costly post-treatments such as strippingor activated carbon.

DETAILED DESCRIPTION OF THE INVENTION

The use of sodium bisulfite in accordance with the decontamination andpurification of water affords an economical, commercially feasible, andconvenient method of adding small amounts of sulfite ion which resultsin the reduced potential formation of total trihalomethanes (TTHM).

The introduction of the sulfite ion into the drinking water purificationprocess can be effected without any interference, or deleterious effect,to the treatment process, which might include the feeding of inorganicchemicals (alum, aluminum chloride, or ferric chloride), or the(organic) coagulant polymers. In some treated waters, where thesetreatment levels are related to the organics in the water, the use ofsodium bisulfite affords substantial economies because of the smalldosages required.

Additionally, a liquid solution of sodium metabisulfite can behomogeneously mixed with liquid alum and other liquid inorganic (AlCl₃,FeCl₃) salts for a commercially feasible combination formula, which canbe fed at a drinking water plant for a single point application. Themixing and transporting of these blended formulas, may be effected atthe point of manufacture by simply adding them together at the time ofloading, prior to shipment. Ratios of mixing sulfite ion and alum, forexample, can be so formulated so as to provide custom blends forindividual drinking water plants. Such ratios are adjusted so as tominimize color, taste and odor in drinking water in processing plants,when metabisulfite is used for this purpose, or as a resultant effect,when the sulfite is used for the control of THM.

The use of the sulfite ion supplying compound in the treatment ofpotable water in accordance with the invention is not intended toreplace chlorine as a disinfectant and purification aid. Rather the useof the sulfite ion is employed so that a reduction in the chlorine feed,for example, to an amount wherein the EPA's MCP limit can be met. Inaccordance with the invention, relatively small dosages of themetabisulfite will produce substantial reductions in the potentialformation of TTHM and a reduced chlorine demand.

In summary the advantages of the invention involving the addition ofsulfite ion include at least the following:

1. A significant reduction in TTHM which results from the pretreatmentof the waters with the sulfite ion prior to chlorination.

2. The successful maintenance of chlorine residual for at least sevendays after treatment when sulfite ion is added.

3. Improved control of bacterial growth by use of sulfite ion inconjunction with chlorination.

4. Non-interference of the sulfite ion with pH or total organic carboncontrol. Durability of the treatment of waters with the sulfite ion.

5. The magnitude of the 7-day TTHM formation potential is not related tothe concentration of metabisulfite applied. For example, sodiummetabisulfite treated water did not catch up to the untreated controlcounterparts in the formation of TTHM over seven days.

6. All three of the variables; time, chlorine dose, and sulfite ionconcentration, independently influenced TTHM formation. The sulfite, aswell as the chlorine, shows significant interaction effects on TTHMformation.

The examples which follow illustrate the essence of the effect of use ofsodium metabisulfite in reducing the formation of total trihalomethanesin water. It is to be understood that these examples are not to beinterpreted as limiting the scope of the invention. Rather it isintended that the scope of the invention be defined by the claimsappended hereto. The examples, and the resulting data, presentedhereinbelow were made with sodium metabisulfite as the source of sulfiteion. However, the advantages of the invention are substantiallyattainable also by employing sulfur dioxide.

EXAMPLES

Solid sodium metabisulfite was added at concentrations of 0, 3.0 and 6.0ppm. Stirring at 40 rpm was commenced for 45 minutes. This contactperiod was intended to simulate the lapsed time for this particularplant between the application of polyelectrolyte and metabisulfite whichwill actually exist during full scale plant testing.

Following the initial 45 minute contact period, the stirrers werebriefly halted to permit the addition of alum at 22 ppm and chlorine atthe appropriate level which is one the normal water treatment process.Mixing was resumed for 30 minutes after which time the samples wereallowed to settle for 3 hours which simulates plant settling. Thesettled waters were filtered through GF/C glass fiber mats using amillipore vacuum filtration apparatus. Upon completion of filtration,chlorine residuals were measured and adjusted to 0.7 ppm. Downwardadjustment was accomplished by adding the theoretical required amount ofmetabisulfite to adjust for any excess chlorine added and upward byadding chlorine.

The pH was measured and recorded following the adjustment of thechlorine residuals. The pH of each solution was then adjusted to 8.4using 0.1 N NaOH. Samples were withdrawn for instantaneous TTHM analysisand transfered to vials containing a few milligrams of sodiumthiosulfate as a part of the analytical procedure to remove any residualchlorine.

Another 40 mL sample was collected for total organic carbon (TOC)analysis, and a 40 mL sample for standard plate count was placed in apre-sterilized plastic bottle. An additional set of samples wascollected from water treatment plant for comparison purposes at theconclusion of the experiments.

The effect of utilizing the metabisulfite is shown in FIG. 1, whichillustrates the instantaneous total trihalomethanes (TTHM) vs. chlorinedose and in FIG. 2, which illustrates the seven-day totaltrihalomethanes vs. metabisulfite dose. It is seen that samples treatedwith 6.0 ppm metabisulfite exhibit a linear relationship with TTHMlevels and chlorine dose in every case. In general, higher TTHM levelsdevelop at higher applied chlorine levels.

The effect of total trihalomethane vs. metabisulfite is indicated byreference to FIG. 3 in which the inverse relationship at the 4 ppmchlorine level for the instantaneous TTHM is illustrated. In the firstround instantaneous TTHM levels dropped from 89 ppb to 42 ppb, while adrop from 126 ppb to 38 ppb was obtained during the second round.

It will be realized that various changes may be made to the specificembodiments shown and described in operating conditions detailed withoutdeparting from the essential concept and spirit of the presentinvention.

I claim:
 1. A method of treating water to disinfect the water and reducethe formation of trihalomethanes which comprises adding to the waterfrom about 0.1 to about 50 parts per million of sulfite ion supplied bya compound selected from the group consisting of sodium metabisulfiteand sulfur dioxide to reduce the chlorine demand and thereafter addingto the water sufficient chlorine to react with any biological materialspresent and maintain a residual amount of chlorine sufficient todisinfect the water and make the water potable, whereby the formation oftrihalomethanes present in the water amounts to less than 100 parts perbillion.
 2. The method of claim 1 wherein the sulfite ion is supplied bysodium metabisulfite.
 3. The method of claim 1 wherein the sulfite ionis supplied by sulfur dioxide.
 4. The method of claim 1 wherein thechlorine is added from about 0.5 to about 24 hours after addition of thesulfite ion supplying compound.
 5. The method of claim 1 wherein theamount of sulfite ion supplying compound added to the water is fromabout 0.5 to about 20 parts per million.
 6. The method of claim 1wherein the formation of trihalomethanes present in the water amounts toless than 80 parts per billion.
 7. The method of claim 1 wherein thetreated water is first coagulated and said sulfite ion is addedsubsequent to the coagulation step.
 8. The method of claim 1 wherein atleast part of the sulfite ion supplying compound is added simultaneouslywith prechlorination.
 9. The method of claim 1 wherein at least part ofthe sulfite ion supplying compound is added intermediate of thechlorination treatment of the water and an additional amount of thesulfite ion supplying compound is added post-chlorination.